1. Field of the invention
The present invention relates to a process for synthesizing oligonucleotides or phosphorothioate oligonucleotides while using a capping agent of N-methylimidazole substantially free of 1,3,5-trimethylhexahydro-1,3,5-triazine.
2. Description of the Related Art
Oligonucleotides and phosphorothioate oligonucleotides are synthesized via coupling of nucleotides. The synthesis generally has the following steps: (a) deblocking, (b) activation/coupling, (c) capping, and (d) oxidation (in the case of oligonucleotides) or sulfurization (in the case of phosphorothioate oligonucleotides). The cycle may be repeated sequentially depending on the number of bases to be coupled.
The capping step is commonly carried out in the presence of a combination of N-methylimidazole and acetic anhydride. In some oligonucleotide syntheses employing N-methylimidazole, it has been observed that some oligonucleotides having unwanted adducts may form. The adducts have been observed to add 85 daltons to the molecular weight of the oligonucleotides.
It would be desirable to have a method for synthesizing oligonucleotides or phosphorothioate oligonucleotides in which the end product is substantially free of unwanted adducts.